Decision criteria for computer-aided parting surface design by B Ravi and M N Srinivasan
A scientific approach is presented and the related logic developed for design of parting surfaces of patterns, moulds and dies used in the manufacture of cast, forged, injection-moulded and die-cast components. This has enabled computer-aided generation of parting surfaces and the determination of projected area, flatness and draw for a parting surface, identification of surfaces to which draft is provided, recognition of component segments causing undercuts, testing for dimensional stability, and location of flash, machined surfaces and feeders. Influencing criteria for parting-surface design have been formulated and developed into algorithms implemented on a personal computer. This approach greatly aids the engineer in rational decision making, paving the way for a systematized code for parting surface design.
Castings are manufactured by pouring molten alloys into various shaped mould assemblies. Moulds are in turn prepared by compacting sand around patterns in segments or halves, one each for the bottom and the top mould. A forged component is manufactured by compressing a heated blank between two shaped dies. In die-casting and plastic injection moulding the material is forced under high pressure into a cavity formed by bringing together two die-halves. In all these manufacturing processes, the design of patterns, moulds and dies, the crucial tooling, directly affects productivity and component quality. The most significant design aspect is the choice of the surface separating the two halves of the mould or die, referred to as the parting surface. A combination of several mechanical, metallurgical and process parameters influences parting-surface location, rendering the design exercise complex.
Decision criteria
Nine influencing parameters for decision making in parting-surface design for a component have been identified: projected area, flatness, draw, draft, undercuts, dimensional stability, flash, machined surfaces and directional solidification.
Projected area
To facilitate removal of the pattern from the mould or the manufactured component from the die, the cross-sectional area should gradually decrease from the parting surface to points farthest from the parting surface. The basal plane of an upright cone or the diametrical planes of a sphere satisfy this requirement. This condition is applicable to flat as well as irregular parting surfaces.
Flatness
Considering technological aspects such as side thrust, dimensional stability, sealing off, flash, and complexity in tooling and mould making, a flat parting surface is preferred over an irregular one.
Draw
Draw is the minimum distance through which a component is linearly translated in order to clear it from the mould. Design considerations such as application of draft to the vertical surfaces of mould and metallurgical and technological problems such as grain flow in forgings, flask size in castings and machine draw capability in injection moulding, as well as increase of cycle time and reduced productivity, are caused by deep draw.
Draft
A conical or pyramidal part is drawn along its axis from a mould with ease compared to a straight cylinder or rectangular part. Surface quality, interracial interaction between the pattern or component and the mould or die, the extent of draw and related factors also contribute to this effect. All such faces of the component that are parallel to the draw vector are given a small taper or draft to aid in easy withdrawal. Surfaces that are not parallel to the draw direction are considered to have a natural draft or form undercuts. Application of draft results in the alteration of the part geometry and additional machining may be required to restore the shape of the component.
Undercuts
Projections or recesses in the component unfavorably inclined with respect to the draw vector hinder the removal of the pattern from the mould or component from the die. It is not always possible to design the parting surface to avoid undercuts completely, so to overcome this, cores or other manufacturing devices like inserts and loose-pieces have to be incorporated. This directly affects the process cycle time and tooling and manufacturing costs.
Dimensional stability
Tile possibility of mismatch between the cope and drag portions of the mould or at the joint between two die halves, results in dimensional reliability across the parting surface being considerably lower compared to that in the portions of the mould lying on one side of the parting surface. Hence the parting surface is designed such that any two points between which high dimensional tolerance is required occur on the same side of it.
Flash
A material flowing into the gaps at the plane of separation of the two mould halves or the interface between mould and a core produces fin-like protrusions or flash. This is generally trimmed after manufacture. However, flash leaves surface imperfections, and in some cases trimming may not be feasible or economically viable. The designer may also specify certain surfaces to be free of flash. Such surfaces must not intersect the parting surface.
Location of surfaces to be machined
Critical surfaces of a cast component requiring machining are preferably located as the bottom or the vertical walls of the mould, which are relatively free of defects. In general, a casting may have several machined faces, all of which cannot be located as the bottom or vertical surfaces in the mould. When several alternative parting surfaces are considered, their relative merits are quantitatively assessed for location of machined surfaces using the criterion proposed below:
Feeders and directional solidification
Solidification of metal in a mould is accompanied by shrinkage, which can manifest in the form of micro pores or cavities. This is prevented by promoting controlled solidification initiating in thin sections to proceed towards thicker sections. The last freezing section is fed by a reservoir of molten metal (a feeder). This introduces definitive conditions on geometric interfaces and interactions between component sections, from which a number of design recommendations on variation of cross section and joining sections have emerged. Geometry of the casting and its disposition in the mould, particularly on access to the feeder, is considered an important parameter. The parting surface is chosen so that the hot spots (regions of mass concentration) are at the top of the casting.
Conclusion
A decision-making scheme on a scientific basis has been developed to assess the design of a parting surface for components manufactured in moulds or dies. Nine different criteria influencing the design have been identified and delineated to allow them to be analyzed by computer. The logic developed has been implemented in algorithms on a PC and tested on typical mechanical components.
A scientific approach is presented and the related logic developed for design of parting surfaces of patterns, moulds and dies used in the manufacture of cast, forged, injection-moulded and die-cast components. This has enabled computer-aided generation of parting surfaces and the determination of projected area, flatness and draw for a parting surface, identification of surfaces to which draft is provided, recognition of component segments causing undercuts, testing for dimensional stability, and location of flash, machined surfaces and feeders. Influencing criteria for parting-surface design have been formulated and developed into algorithms implemented on a personal computer. This approach greatly aids the engineer in rational decision making, paving the way for a systematized code for parting surface design.
Castings are manufactured by pouring molten alloys into various shaped mould assemblies. Moulds are in turn prepared by compacting sand around patterns in segments or halves, one each for the bottom and the top mould. A forged component is manufactured by compressing a heated blank between two shaped dies. In die-casting and plastic injection moulding the material is forced under high pressure into a cavity formed by bringing together two die-halves. In all these manufacturing processes, the design of patterns, moulds and dies, the crucial tooling, directly affects productivity and component quality. The most significant design aspect is the choice of the surface separating the two halves of the mould or die, referred to as the parting surface. A combination of several mechanical, metallurgical and process parameters influences parting-surface location, rendering the design exercise complex.
Decision criteria
Nine influencing parameters for decision making in parting-surface design for a component have been identified: projected area, flatness, draw, draft, undercuts, dimensional stability, flash, machined surfaces and directional solidification.
Projected area
To facilitate removal of the pattern from the mould or the manufactured component from the die, the cross-sectional area should gradually decrease from the parting surface to points farthest from the parting surface. The basal plane of an upright cone or the diametrical planes of a sphere satisfy this requirement. This condition is applicable to flat as well as irregular parting surfaces.
Flatness
Considering technological aspects such as side thrust, dimensional stability, sealing off, flash, and complexity in tooling and mould making, a flat parting surface is preferred over an irregular one.
Draw
Draw is the minimum distance through which a component is linearly translated in order to clear it from the mould. Design considerations such as application of draft to the vertical surfaces of mould and metallurgical and technological problems such as grain flow in forgings, flask size in castings and machine draw capability in injection moulding, as well as increase of cycle time and reduced productivity, are caused by deep draw.
Draft
A conical or pyramidal part is drawn along its axis from a mould with ease compared to a straight cylinder or rectangular part. Surface quality, interracial interaction between the pattern or component and the mould or die, the extent of draw and related factors also contribute to this effect. All such faces of the component that are parallel to the draw vector are given a small taper or draft to aid in easy withdrawal. Surfaces that are not parallel to the draw direction are considered to have a natural draft or form undercuts. Application of draft results in the alteration of the part geometry and additional machining may be required to restore the shape of the component.
Undercuts
Projections or recesses in the component unfavorably inclined with respect to the draw vector hinder the removal of the pattern from the mould or component from the die. It is not always possible to design the parting surface to avoid undercuts completely, so to overcome this, cores or other manufacturing devices like inserts and loose-pieces have to be incorporated. This directly affects the process cycle time and tooling and manufacturing costs.
Dimensional stability
Tile possibility of mismatch between the cope and drag portions of the mould or at the joint between two die halves, results in dimensional reliability across the parting surface being considerably lower compared to that in the portions of the mould lying on one side of the parting surface. Hence the parting surface is designed such that any two points between which high dimensional tolerance is required occur on the same side of it.
Flash
A material flowing into the gaps at the plane of separation of the two mould halves or the interface between mould and a core produces fin-like protrusions or flash. This is generally trimmed after manufacture. However, flash leaves surface imperfections, and in some cases trimming may not be feasible or economically viable. The designer may also specify certain surfaces to be free of flash. Such surfaces must not intersect the parting surface.
Location of surfaces to be machined
Critical surfaces of a cast component requiring machining are preferably located as the bottom or the vertical walls of the mould, which are relatively free of defects. In general, a casting may have several machined faces, all of which cannot be located as the bottom or vertical surfaces in the mould. When several alternative parting surfaces are considered, their relative merits are quantitatively assessed for location of machined surfaces using the criterion proposed below:
Feeders and directional solidification
Solidification of metal in a mould is accompanied by shrinkage, which can manifest in the form of micro pores or cavities. This is prevented by promoting controlled solidification initiating in thin sections to proceed towards thicker sections. The last freezing section is fed by a reservoir of molten metal (a feeder). This introduces definitive conditions on geometric interfaces and interactions between component sections, from which a number of design recommendations on variation of cross section and joining sections have emerged. Geometry of the casting and its disposition in the mould, particularly on access to the feeder, is considered an important parameter. The parting surface is chosen so that the hot spots (regions of mass concentration) are at the top of the casting.
Conclusion
A decision-making scheme on a scientific basis has been developed to assess the design of a parting surface for components manufactured in moulds or dies. Nine different criteria influencing the design have been identified and delineated to allow them to be analyzed by computer. The logic developed has been implemented in algorithms on a PC and tested on typical mechanical components.
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